Biaxially oriented polyester films have been conventionally used as a forming film for use in molding a transfer foil (for example, see Japanese Patent Application Laid-Open Nos. 6-210799 and 2000-344909), but formability for transferring the foil to a material having a large drawing ratio or a complicated shape is insufficient when the biaxially oriented polyester films are used in forming use, e.g., for use as transfer foils.
In addition, as a polyester film for the improvement of formability, a transfer foil using a copolymerized polyester film with a lower forming stress compared with a biaxially oriented polyester film (for example, see Japanese Patent No. 3090911) and a polyester film directed for use in formed, processed, and printed products, containing a specific glycol component (e.g., butanediol) as a glycol component constituting a polyester (for example, see Japanese Patent Application Laid-Open No. 2002-97261) have been proposed. However, transfer foils using these polyester films are easily whitened by an organic solvent contained in a printing ink, such as ethyl acetate, methyl ethyl ketone, toluene, acetone and the like and, thus, the transparency and smoothness of the film surface become deteriorated, causing a problem of easy occurrence of printing defects because printability is not considered particularly although formability is good. Accordingly, a film having a solvent resistance to various solvents contained in a printing ink, i.e., a film excellent in printability, has been demanded.
Further, although a non-oriented polyester film has a good formability compared to a biaxially oriented film, the film is sequentially coated with a release film layer, a top layer, and a printing layer when it is used, for example, for forming uses such as transfer foils. Each layer can be sequentially formed by applying a coating solution dissolved in a solvent for the formation of these layers, such as ethyl acetate, methyl ethyl ketone, toluene, acetone and the like or a mixture thereof, and drying the solvent. The temperature to dry the solvent under heating is near the boiling point of each solvent, or at or above the boiling point of each solvent (ethyl acetate (boiling point: 77° C.), methyl ethyl ketone (boiling point: 80° C.), toluene (boiling point: 110° C.), acetone (boiling point: 56° C.)). However, compared to the biaxially oriented film, the non-oriented polyester film is easy to produce a transformation such as wrinkles, elongation, shrinkage and the like when a tension is applied at a drying temperature which is at or above the glass transition temperature because the molecules are not fixed due to such an stretching and thermal treatment, as well as when it is transferred on a molded product, a printed matter is transformed to cause a molding defect, resulting in a problem that the heat resistance to 80 to 120° C. is insufficient.
Moreover, a polyester sheet for molded part obtained by laminating a thermoplastic polyester having a melting point of 180° C. or higher and a crystallization parameter Δ Tcg (crystallization temperature-glass transition temperature) of 50° C. or lower on both sides of another thermoplastic polyester has been proposed (for example, see Japanese Patent No. 3139512). However, heat resistance and solvent resistance of the film are insufficient because the thickness of the multilayer laminate and the crystallinity on the surface of the multilayer laminate have not been considered and, thus, such a film does not satisfy all of the heat resistance, solvent resistance, and formability.
Furthermore, non-oriented polyester films are an amorphous film obtained by rapid cooling to a glass transition temperature or lower, followed by solidification upon cooling. These amorphous films are thermodynamically in the non-equilibrium state, and thermodynamic quantities such as volume and enthalpy in a range of from room temperature to the glass transition temperature are relaxed to cause physical degeneration without chemical changes. It is said that a heterogeneous structure is generated in the amorphous film due to the enthalpy relaxation or volume relaxation, and such a heterogeneous structure as a fault causes embrittlement. On the other hand, because a biaxially oriented film is crystallized or oriented by stretching or heat-set, the molecule is difficult to move and these relaxation phenomena do not occur. The thinner the amorphous film thickness is, the more easily the relaxation phenomenon progresses. The thicker the film thickness is and the longer the length of the molecule is (the larger the intrinsic viscosity of the film is), the more the relaxation phenomenon occurs although its progress is delayed. Specifically, an elongation at break of the film immediately after its production shows a few hundred percent, but the elongation at break after the passage of time is a few percent, and when various processings such as slit formation are performed, break and tear easily occur, leading to bad handling. Accordingly, improvements have been demanded for this embrittlement over time.
Therefore, a film which can satisfy all of flexibility, heat resistance, solvent resistance, printability, formability, and embrittlement resistance over time has been desired.
It could therefore be helpful to provide a multilayer polyester film including a polyester (B) layer mainly composed of a polyester (B) laminated on at least one side of a polyester (A) layer mainly composed of a polyester (A), wherein the flexibility and embrittlement resistance over time of the film are retained by specifying the elastic modulus of the multilayer film in an atmosphere at 23° C. and 120° C., and the heat resistance, solvent resistance, printability, and formability are also retained by defining the thickness of the polyester (B) layer within a specific range as well as defining a face orientation coefficient within a specific range.